“Here’s something you don’t see in every Ben Affleck/Liv Tyler movie: the stars of “Armageddon” performing a musical number from Stephen Sondheim’s savage musical, “Sweeney Todd.” It happens toward the end of Kevin Smith’s “Jersey Girl,” and it’s one of the few unexpected touches in a sentimental, overly familiar comedy-drama about the joys of parenthood, the frustrations of workaholic dads and the necessity of spending time with the kids. Affleck and Tyler end up performing the number in order to please his 7-year-old daughter, Gertie (Raquel Castro), who falls in love with the show when dad takes her to a Broadway performance. She chooses to recreate the experience at her school, recruiting them to perform it with her. The song is handled with such affection and style that it suggests the writer-director of “Clerks” and “Dogma” might just be a closet Sondheim fan. Or maybe it’s just because he can’t stand “Cats,” which is the clear favorite with Gertie’s classmates — and the target of several barbs in the script. [www.phillyburbs.com]